Surface-micromachined absolute pressure sensor and a method for manufacturing thereof

ABSTRACT

A capacitive pressure sensor structure, in particular for measurement of absolute pressure, and a method for manufacturing the sensor. The sensor includes at least one fixed electrode, and at least one movable electrode electrically isolated from said fixed electrode and spaced apart from said fixed electrode. A portion of said movable electrode is formed from a porous polycrystalline silicon layer that in a finished component remains as an integral portion of said flexibly movable electrode.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/FI01/00970 which has an Internationalfiling date of Nov. 7, 2001, which designated the United States ofAmerica.

The invention relates to a capacitive pressure sensor according to thepreamble of claim 1 and a method for manufacturing the same.

Traditionally, micromechanical pressure sensors are categorized in twoclasses according to their manufacturing method. A pressure sensor iscategorized as a surface micromechanical sensor if it is manufacturedusing surface micromechanical techniques, while the term bulkmicromechanical device is used if the fabrication of the sensor is basedon the older bulk micromechanical technique.

On the basis of their constructional differences, pressure sensors arealso categorized in two classes depending on whether the sensor isresponsive to a differential pressure or an absolute pressure. Thepresent patent application discloses a novel construction for a surfacemicromechanical absolute pressure sensor and a method for manufacturingthe same.

Prior-art sensor structures are described, e.g., in publication K.Kasten et al.; Sensors and Actuators A, Vol. 85 (2000), pp. 147-152. Inthe sensor structure taught herein, the bottom electrode is formed fromsingle-crystal silicon layer on a SIMOX substrate. The top electrode ofthe structure is made from polycrystalline silicon. The so-calledsacrificial layer needed during fabrication is entirely removed viachannels located at the edges of the electrodes. Next, the openingsremaining from the etching of the sacrificial oxide layer during themanufacturing process are closed by depositing thereon silicon nitrideusing the PECVD process, whereby between the capacitive electrodes isformed a partial vacuum determined by the pressure of the PECVD process.

The shortcomings of the structure proposed by Kasten are obvious. Sincethe sacrificial layer is entirely etched away from the interior of thestructure, a step discontinuity will remain on the flexible diaphragm atthe edges of the sacrificial layer. Hence, the tensional stress of theflexible diaphragm must be controlled very low to prevent the inherenttensional stress of the diaphragm from bending it into contact to thebottom electrode. Resultingly, the capacitive elements must be made verysmall. Since a practicable sensor construction needs a capacitance inthe order of 10 pF, a large number of capacitive elements is required.As a consequence, the relative proportion of the variable capacitancewith regard to the overall capacitance remains low because of theelement edge structures that in a large number of small elementsincrease the proportion of stray capacitance higher than what isachievable in such a sensor construction that facilitates the use of alarger capacitive element.

Also the reference structure in the absolute pressure sensorconstruction proposed by Kasten is problematic. Therein, the stiffeningof the reference element electrode is solved by way of leaving onto thereference elements a thick layer of oxide deposited in the LPCVDprocess. Since the thermal expansion constants of elemental silicon andits oxide are different from each other, this arrangement may beexpected to impart a higher temperature dependence to the referenceelement structure. As a rule, the temperature/humidity dependencies ofthe sensing and reference structures, respectively, should be as equalas possible.

It is an object of the present invention to overcome the problems ofprior-art techniques and to provide an entirely novel type of absolutepressure sensor.

The goal of the invention is achieved by way of using a porouspolycrystalline silicon layer as a portion of the flexible diaphragm ofthe sensor structure.

More specifically, the absolute pressure sensor according to theinvention is characterized by what is stated in the characterizing partof claim 1.

Furthermore, the method according to the invention is characterized bywhat is stated in the characterizing part of claim 10.

The invention offers significant benefits.

The invention makes it possible to implement pressure sensors coveringneeds from the barometric range up to pressures in the order of hundredsof bars. The size of individual capacitive elements is sufficientlylarger even at barometric pressures to keep the relative proportion ofstray capacitances reasonably low. The area of the sacrificial layer tobe etched away can be defined by lithography techniques, whereby themanufacturing tolerances are improved. A portion of the oxide acting asthe sacrificial layer can be left unetched. As a consequence, theflexible diaphragm retains a straight shape at its edges. Resultingly,the internal tensional stress of the diaphragm can be adjusted high thusfacilitating the use of a larger element size. The reference structureaccording to the invention is responsive to changes in ambienttemperature and, e.g., humidity in the same manner as thepressure-responsive structure proper. Hence, the overall construction ofthe sensor can be made very stable and secondary factors causing extratemperature drift and dependence on ambient humidity can be readilycompensated for.

In the following, the invention will be examined in greater detail withthe help of exemplifying embodiments illustrated in the appendeddrawings in which

FIG. 1 is a cross-sectional view of a sensor element according to theinvention;

FIG. 2 is a top view of a capacitive pressure sensor comprised ofelements (25 pcs.) shown in FIG. 1;

FIG. 3 is a cross-sectional view of a reference sensor element accordingto the invention;

FIG. 4 is a top view of the reference element area of a capacitivepressure sensor according to the invention;

FIG. 5 is a schematic top view of a complete capacitive pressure sensoraccording to the invention;

FIG. 6 is a cross-sectional view of an alternative embodiment of asensor element according to the invention;

FIG. 7 is a cross-sectional view of a second embodiment of a sensorelement according to the invention; and

FIG. 8 is a schematic list of manufacturing steps in the fabricationmethod of a sensor according to the invention.

Referring to FIG. 2, a sensor element of an absolute pressure sensoraccording to the invention comprises a polycrystalline silicon layer 3which is deposited on a dielectric layer 2 made on a silicon substrate1, is doped conductive and has another dielectric layer 4 depositedthereon. Over these layers is deposited a polycrystalline silicon layer5 having thereon deposited a polycrystalline silicon layer 6 containingsmall-diameter pore holes in abundance. Layer 5 is optional in theoverall structure. It may be omitted if the definition of thesacrificial layer takes place by lithography technique after thedeposition of the porous silicon layer. Layer 5 may also extend into theregion above area 10. Then, the layer has one or more openingspermitting layer 6 to communicate with area 10. Above the porous siliconlayer is again deposited a uniform silicon layer 7 that serves as asubstrate for metallization layer 8. Dielectric layer 4 andpolycrystalline silicon layer 5 are removed from the center and edgeareas of the capacitive element. At the edge area of the element, ametallization layer 9 is deposited onto the conductive polycrystallinesilicon layer. With the exception of the contact areas and the flexiblediaphragm, area 10 of the sensor structure is covered by a passivationlayer 11.

Dielectric layer 2 is most advantageously silicon dioxide with athickness of 500-2000 nm typical. Polycrystalline silicon layer 3 ismade conductive by doping with phosphorus or boron, for instance. Layer4 is made of a dielectric material, most advantageously silicon dioxide.Layer 5 is most advantageously made of doped polycrystalline silicon.Layer 6 is a porous, doped polycrystalline silicon layer having athickness of about 100 nm. The conductive polycrystalline silicon layer7 deposited on layer 6 is typically 100 nm to 5000 nm thick. Incombination with the internal tensional stress of the capacitiveelement, the thickness of layer 7 plays a crucial role in thedimensioning of the sensor element. An example on the depositiontechnique of the porous silicon layer (with a high density of holes) isdescribed, e.g., in publication Y. Kageyama, T. Tsuchiya, H. Fuanbashi,and J. Sakata: “Polycrystalline silicon thin films with hydrofluoricacid permeability for underlying oxide etching and vacuum encapsulation”J. Vac. Sci Technol. A 18(4), July/August 2000. An essential factor inthe structure of layer 6 is that its pores are very small (with anaverage minimum diameter of less than 10 nm).

The metallization layers 8 and 9 make an electrical contact to layers 3and 7. Most advantageously, metallization layers 8 and 9 are made bysputtering a 1000 nm thick aluminum layer. The sensor cavity defined byarea 10 in the center region of the structure is at a partial vacuum.The deflection of layers 6 and 7 is determined by the differentialpressure between the cavity area 10 and the ambient pressure. The shapeand size of area 10, in combination with the thickness and tensilestress of layers 6 and 7, determine the usable pressure range of thesensor.

Passivation layer 11 that forms the uppermost layer of the structure ismost advantageously made of silicon nitride or using a multilayerstructure of silicon nitride and silicon dioxide. Typically, passivationlayer 11 is about 500 nm thick.

In the pressure sensor embodiment shown in FIG. 2, the bottom electrodesof all the capacitive sensor elements are connected in parallel at acontact area located in the right lower edge of the sensor.Respectively, all the top electrodes are connected by the metallizationlayer at a contact area located in the left upper edge of the sensor.Hence, the capacitance between the bottom electrode contact and the topelectrode contact is the overall capacitance of all the capacitiveelements whose value is dependent on the differential pressure betweenthe ambient pressure-transmitting medium and the internal volume of thesensor taken to a partial vacuum. Resultingly, a measurement of theoverall capacitance is sufficient to determine the ambient pressure onthe basis of the sensor calibration data.

In addition to the variable capacitance of the active area, the overallcapacitance measurable across a sensor invariably includes an inherentcapacitance of the sensor structure known as the stray capacitance. As arule, the value of stray capacitance measured over a sensor structure isdependent on the component temperature and, e.g., ambient humidity.Since it is generally impossible to arrange a measurement over a singlecomponent such that the portion of the variable capacitance is detectedseparately from the stray capacitance, it is advantageous to integrateon the same silicon chip also a separate structure that can be used foreliminating the contribution of stray capacitances on the measured valueof pressure. Such a reference structure of non-pressure-responsivecapacitance is most preferably constructed as identical as possible tothe pressure-responsive measuring sensor as to its capacitancedependence on ambient parameters (e.g., temperature and humidity).

To a person versed in the art it is obvious that the referencecapacitance may also be constructed on a separate silicon chip, wherebythe integration of a pressure sensor with a complete reference can beimplemented using a suitable packaging technology.

In FIG. 3 is shown a cross-sectional view of the structure of areference element according to the invention. As is evident from thediagram, the sacrificial layer contains within area 10 a number ofcolumn pads 14 formed by layers 4 and 5. The purpose of the pads is tostiffen the top electrode so as to eliminate the pressure responsivenessof the reference sensor capacitance. Typically, the circular column padsneeded in the structure have a diameter of 1 μm to 10 μm. The number ofpads serving to stiffen the top electrode formed by layers 6 and 7(thereby reducing its pressure responsiveness) may be varied from 1 to100 per capacitive element. As to the dimensioning of the sensor, it isessential that the overall area of column pads formed on area 10 of asingle capacitive element of the reference sensor is substantiallysmaller than the overall area of the sensor element, whereby the elementof the reference sensor is as identical as possible to the correspondingelement of the actual pressure-responsive sensor except for itsnon-responsiveness to pressure variations.

Other techniques for reducing the responsiveness of the reference sensorto pressure can be found through increasing the thickness of layer 6 aswell as from increasing the internal tensional stress of layers 6 and 7in regard to that of the actual pressure-responsive elements.

In FIG. 4 is shown a top view of the reference area of a pressuresensor. The reference elements shown in the diagram have 16 supportingcolumn pads over the area 10 of the sacrificial layer etched away placedso as to stiffen the diaphragm formed by layers 6 and 7.

To a person versed in the art it is obvious that the structure may becomplemented by placing between the top and bottom electrodes anadditional electrode known as a guard electrode that may be used, e.g.,for eliminating measurement errors caused by surface leakage currents.Respectively, a person versed in the art is fully aware that thelowermost polycrystalline silicon layer (bottom electrode) 3 and thedielectric layer 2 may be omitted from the structure if it is desirableto use the silicon substrate alone as the bottom electrode.

Alternative Embodiment of the Structure

The relative proportion of the pressure-responsive capacitance in regardto the overall capacitance may be increased by making the elementsthicker at their center areas. Thus, the layer subject to flexure underan external pressure flexes in the structure more at its edges than atits central area. As the central area remains at any pressureessentially flat over the entire span of pressure measurement, theproportion of the pressure-responsive capacitance in regard to theoverall capacitance is resultingly increased. In a practicablerealization of the sensor, the thicker portion 12 shown in FIG. 6 ismost preferably made from polycrystalline silicon (analogously to layer7).

The thicker area of the flexible diaphragm may also be made prior to thedeposition of the porous polycrystalline silicon layer. However, thearea must then be patterned with openings to achieve efficientetching-away of the sacrificial layer thereunder.

The basic structure (FIG. 3) may also be improved by extending layer 5over the entire area of the flexible diaphragm. In FIG. 7 is shown across-sectional view of such a variant of the structure. In thisconstruction, the stiffness of the diaphragm is improved by a layer 5that most preferably is of the same material as layer 7 (that is,polycrystalline silicon). For proper etching-away the sacrificial layerin this embodiment of the sensor structure, layer 5 must be providedwith one or more openings 13.

As an alternative embodiment of the sensor construction may also beinterpreted such a structure wherein the lowermost polycrystallinesilicon layer (bottom electrode) 3 and dielectric layer 2 are omitted.Then the bottom electrode may be formed by the silicon substrate 1 thatis doped conductive.

Dimensioning of the Sensor

The sensor can be dimensioned (as to the layer thicknesses, geometry andtensional stresses) using commercially available FEM software. In thesimplest case using circular areas as the flexible portions of thesensor elements, the dimensioning thereof can be performed with the helpof the analytical tools [3] written below.

Flexure of a circular diaphragm (drum diaphragm):${{Z(r)} = {\frac{p \cdot R^{2}}{4 \cdot \sigma \cdot h}\left\lbrack {1 - \frac{r^{2}}{R^{2}}} \right\rbrack}},$where p=pressure

-   -   R=radius of etched-free diaphragm area    -   σ=tensional stress of diaphragm    -   h=thickness of flexible diaphragm    -   r=distance from diaphragm center point

EXAMPLE

-   -   P=1·10⁵ Pa    -   h=1 μm (thickness of flexible polycrystalline silicon layer)    -   σ(at 3000 ppm strain)=0.03·160·10⁹ Pa (tensional stress of        diaphragm after thermal treatment of polycrystalline silicon        diaphragm at 700° C., 1 h);    -   equation solving as Z(0)=0.5 μm when R=98 μm (whereby deflection        of the diaphragm is half the sensor air gap at a pressure of        1·10⁵ Pa and sacrificial layer thickness d_(u) of 1 μm).

With the values of the example, about 25 elements fit on 1 mm².

Overall capacitance of sensor (having N elements) as a function ofpressure:$C_{e} = {\int_{0}^{R}{\frac{2 \cdot N \cdot ɛ \cdot \pi \cdot r}{d_{u} - {\frac{p \cdot R^{2}}{4 \cdot \sigma \cdot h}\left( {1 - \frac{r^{2}}{R^{2}}} \right)}}\quad{\mathbb{d}r}}}$

Referring to FIG. 8, the method comprises the following steps in orderof execution:

1. Oxide layer growth

2. Amorphous silicon layer growth (complemented with, e.g., in situdoping with boron)

3. Thermal treatment

4. Patterning of polycrystalline silicon layer

5. Deposition of sacrificial layer (LTO)

6. Amorphous silicon layer growth (complemented with, e.g., in situdoping with boron)

7. Amorphous silicon layer patterning

8. Amorphous silicon layer growth (doping with boron, made porous/withopenings, thickness 100 nm)

9. Thermal treatment

10. Sacrificial layer etching

11. Polycrystalline silicon layer growth (complemented with, e.g., insitu doping with boron)

12. Polycrystalline silicon layer patterning

13. LTO layer patterning

14. Metallization layer deposition

15. Metallization layer patterning

16. PECVD growth of silicon nitride layer

17. Patterning of PECVD grown silicon nitride layer.

As a rule, porous layer 6 must understood as a layer pervious to etchingchemicals but impervious to the materials of layer 7 that issubsequently deposited thereon.

References

-   -   1. K. Kasten et al.: Sensors and Actuators A, Vol. 85 (2000),        pp. 147-152.    -   2. Y. Kageyama, T. Tsuchiya, H. Fuanbashi, and J. Sakata:        “Polycrystalline silicon thin films with hydrofluoric acid        permeability for underlying oxide etching and vacuum        encapsulation,” J. Vac. Sci. Technol. A 18(4), July/Aug. 2000.    -   3. George S. K. Wong et al.: AIP Handbook of Condenser        Microphones, Theory, Calibration and Measurements, AIP Press,        New York, 1995, pp. 41-42.

1. A capacitive pressure sensor structure, in particular for measurementof absolute pressure, the sensor comprising: at least one fixedelectrode; a dielectric layer formed on top of the at least one fixedelectrode, a central portion of the dielectric layer being removed toform a space exposing a central portion of the at least one fixedelectrode; and at least one movable electrode mounted on top of thedielectric layer and extending across the space, the at least onemovable electrode thereby being electrically isolated from and separatedfrom said at least one fixed electrode, wherein the space is partiallyvacuumed, and wherein a portion of said at least one movable electrodeis formed from a porous polycrystalline silicon layer that is anintegral portion thereof.
 2. The sensor structure of claim 1, wherein astructure remaining under edges of said porous polycrystalline siliconlayer is stiffened by providing thereunder a uniform silicon layerhaving openings made thereto.
 3. The sensor structure of claim 1,wherein columnar pads are formed within the space thereby forming areference sensor element.
 4. The sensor structure of claim 1, wherein acentral area of the at least one movable electrode includes a thickportion that serves to stiffen the at least one movable electrode at thecentral area thereof.
 5. The sensor structure of claim 1, furthercomprising a polycrystalline silicon layer that is impervious to gasdeposited over the porous polycrystalline silicon layer so that the atleast one movable electrode becomes impervious to gases.
 6. The sensorstructure of claim 1, wherein the sensor structure constitutes aplurality of single sensor elements connected in parallel or series witheach other.
 7. The sensor structure of claim 1, wherein the sensorstructure is manufactured using silicon micromechanical techniques. 8.The sensor structure of claim 1, wherein the sensor structure is anabsolute pressure sensor.
 9. A method of manufacturing a capacitivepressure sensor structure, in particular a sensor for measurement ofabsolute pressure, the sensor comprising the steps of: providing atleast one fixed electrode; providing a dielectric layer on said at leastone fixed electrode; etching-away predetermined portions of a centralportion of the dielectric layer to form a space exposing a centralportion of the at least one fixed electrode, the dielectric layer beingetched so that a plurality of columnar pads remain in the centralportion of the dielectric layer and edge portions of the dielectriclayer remain around the space, and providing at least one movableelectrode on top of the dielectric layer, the at least one movableelectrode being electrically isolated from said at least one fixedelectrode by the space, characterized in that wherein a portion of saidat least one movable electrode is formed from a porous polycrystallinesilicon layer that is an integral portion thereof.
 10. The method ofclaim 9, wherein during the step of etching the dielectric layer, theedge portions of the dielectric layer are left byway of not carrying thethe etching step to completion in a lateral direction.
 11. The method ofclaim 9, further comprising the step of providing a uniform siliconhaving openings under the porous polycrystalline silicon layer in orderto stiffen the porous polycrystalline silicon layer.
 12. The method ofclaim 9, wherein a central portion of the at least one movable electrodeincludes a thick area that serves to stiffen the at least one movableelectrode at the central area thereof.
 13. The method of claim 9, apolycrystalline silicon layer that is impervious to gas over the porouspolycrystalline silicon layer such that the at least one movable layerbecomes impervious to gases.
 14. The method of claim 9, wherein thesensor structure constitutes a plurality of single sensor elementsconnected in parallel or series with each other.
 15. The method of claim9, wherein the sensor structure is manufactured using siliconmicromechanical techniques.
 16. The method of claim 9, furthercomprising the step of creating a partial vacuum in the space via theporous polycrystalline silicon layer.
 17. The method of claim 9, furthercomprising the step of sealing the sensor structure by depositing ontosaid porous polycrystalline silicon layer another polycrystallinesilicon layer in a process that leaves a partial vacuum in the space inan interior of the sensor structure.